Deviations from Mendel’s Findings

Deviations from Mendel’s Findings 

  Few generalizations were arrived at by Mendel, based on his experiments of garden pea plant- such as,  

• Single trait Single gene Two alleles.  

• Two alleles show interaction in which one is completely dominant.  

• Factors (genes) for different traits present on different chromosomes assort independently. 

Types of gene interaction: 

1. Intragenic interactions: It occur between the alleles of same gene e.g. incomplete dominance and co-dominance. It also occurs between the multiple allele series of a gene.

2. Intergenic (non-allelic) interactions: It occur between the alleles of different genes present on the same or different chromosomes. e.g. pleiotropy, polygenes, epistasis, supplementary and complementary genes, etc. Some of these interactions are discussed below

a. Incomplete dominance:  

• In incomplete dominance, both the alleles (genes) of an allelomorphic pair express themselves partially. One allele (gene) cannot suppress the expression of the other allele (gene) completely. 

• A well-known example is the flower colour of Mirabilis jalapa. If a red-flowered (RR) plant is crossed with a white-flowered (rr) plant, then F1 offsprings have pink (Rr) flowers. 

 F2 Generation: Selfing of F1 

Result: Genotypic ratio - 1RR : 2Rr : 1rr  

Phenotypic ratio - 1Red: 2 Pink :1 White 

b. Co-dominance:  

• In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in F1 hybrids. Such alleles which are able to express themselves equally independently in hybrids, are called co-dominant alleles. Thus in co-dominance both alleles are expressed. 

• A classic example of co-dominance is coat colour in cattle. There are two types one with red coat (with red colour hair) and other with white coat (with white hair).  

• When red cattles (RR) are crossed with white cattles (WW), F1 hybrids (RW) are roan. Roans have the mixture of red and white colour hair.  

• Thus both the traits are expressed equally. In F2 generation red (RR), roans (RW) and white (WW) are produced in the ratio 1:2:1. Thus in Co-dominance, the genotypic and phenotypic ratios are identical. 

 Representation of co-dominance in cattle 

c. Multiple alleles:   

• More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue, are known as multiple alleles. 

• In Drosophila, a large number of multiple alleles are known. e.g. The size of wings from normal wings to vestigial (no) wings, i.e., just stumps, is due to one allele (vg) in homozygous condition.
• The normal wing is wild type while vestigial wing is recessive type. 

 Few phenotypes and genotypes in Drosophila 

d. Pleiotropy:  

• When a single gene controls two (or more) different non-related traits, it is called pleiotropic gene and the phenomenon is called pleiotropy or pleiotropism. 

• The disease, sickle-cell anaemia, is caused by a gene Hbs . Normal or healthy gene HbA is dominant. The carriers (heterozygotes HbA/Hbs ) show signs of mild anaemia as their RBCs become sickle-shaped.  

• A marriage between two carriers will produce normal, carriers and sickle-cell anaemic children in 1:2:1 ratio. Sickle cell anaemics die leaving carriers and normals in the ratio 2:1. 

 Representation of Pleiotropy